In order to obtain accurate readings from a flow meter, it must be calibrated periodically by determining its characteristic or K-factor. The K-factor is a constant of proportionality between the flow rate of the fluid flowing through the flow meter and the response provided by the flow meter to the flow rate. A typical turbine type flow meter develops electrical oscillations proportional in number to the volume of flow through the flow meter. The characteristic is expressed in terms of the number of pulses generated by the flow meter per unit volume of fluid passing through the flow meter. Moreover, the characteristic is a function of the type of fluid as well as the fluid temperature, pressure, flow rate and varies as the flow meter parts wear in the course of use. An apparatus for determining the characteristic of a flow meter while in an operating fluid system is called a ‘prover.’ An apparatus for determining the characteristic of a flow meter on a test stand and not in a fluid system is called a ‘calibrator.’
It is well known to determine the characteristic of a flow meter by comparing its response to a ballistic flow calibrator or prover connected in series with the flow meter. A prover uses a piston that travels in a cylindrical chamber in synchronism with a fluid traveling through the flow meter. By measuring the time interval required for the piston to travel through a known volume of the chamber, an average flow rate can be calculated. These calculations may be used to determine the characteristic (K-factor) of the flow meter.
In existing provers, the release and return of the piston involves many difficult mechanical problems which have not been regularly overcome. These mechanisms tend to be complex and the prover itself bulky and costly to construct. Some provers utilize complex reverser valves to reverse the direction of flow in a cylinder and thereby return the piston to its original position. Other embodiments utilize devices to retract a piston and restrain it in the upstream position or bypass the flow of the piston by means of a poppet or bypass valve when the prover is not being used in a proving test. Provers utilizing valves to reverse the direction of flow are known as ‘bi-directional provers’ because the proving test may be made with the piston traveling in either direction. Provers utilizing devices to retract and restrain the piston are known as ‘unidirectional provers’ because the fluid and piston always travel in the same direction in the cylinder during a test.
Numerous patents have issued disclosing various types of provers. U.S. Pat. No. 3,492,856 discloses a ballistic flow calibrator in which the piston has a passage through it. A valve seals the passage when it is closed and permits fluid flow through the piston when it is opened. U.S. Pat. No. 4,152,922 discloses a ballistic flow calibrator with an auxiliary piston and an auxiliary cylinder to control a fluid displacement measuring piston which moves through a fluid measuring cylinder as a fluid barrier the same distance as the auxiliary piston moves through the auxiliary cylinder. U.S. Pat. No. 3,492,856 describes a unidirectional flow meter calibrating apparatus employing a piston within a conduit where the piston is restrained in the upstream position by means of a complex motor, clutch and cable assembly located upstream of the conduit. A poppet valve, held open by the cable, provides a fluid passage through the piston when the apparatus in not being used for flow measurements. Releasing the cable permits fluid pressure to close the poppet valve setting the piston in motion. U.S. Pat. No. 4,152,922 discloses a prover in which a measuring piston is returned and restrained in its upstream position by means of a second control piston. The control piston travels through a separate control cylinder and is linked to the measuring piston by a rod. A source of pressurized air is used to move the control piston. U.S. Pat. No. 4,794,783 discloses a similar prover wherein the control cylinder is moved by pressurized hydraulic fluid.